The field of optics has gained a great deal from the invention of the confocal microscope by Marvin Minsky. This device is described in U.S. Pat. No. 3,013,467. The confocal microscope attains very high resolution because it focuses a point-source signal on a specimen and images the reflected light as a point using a pin-hole aperture. All out-of-focus signals are eliminated by the aperture. Of course, this arrangement limits the microscope's field of view. Thus, the examination of a typical object requires that either the object or the focal point be scanned. The data gathered during a scan can be used to generate a three-dimensional image.
Confocal microscopes can have one of two basic geometries. The first is called the "double focusing" arrangement and it uses two lenses and two apertures. The first aperture produces the point-source signal, which is focused by the first lens on the object. The light passed through the object is focused by the second lens through the second aperture. The second, called the "reciprocal" arrangement, takes advantage of the same lens for focusing the light on the object and focusing the reflected light. A beam splitter is interposed between the light source and the lens such that the reflected signal is deflected to a separate aperture and detection system. Both types are discussed by Minsky and in general literature.
At the present time confocal microscopes are employed in biological, medical, semiconductor and industrial applications. These microscopes rely on a laser light source rather than an apertured extended source to produce the necessary light signal. Additionally, computers and video systems are used to process, store and display detected images. As an example, confocal microscopes are used in systems for optical inspection of silicon wafers and lithographic masks and for the inspection of disks used in data storage. A variety of further applications and correspondingly modified confocal systems are described in the following U.S. Pat. Nos. 5,091,652; 5,162,942; 5,283,433; 5,283,684; 5,296,703; 5,351,152 and 5,581,345.
Due to the large dimensions and cumbersome geometries which limit the applicability of conventional confocal devices and systems there is a need to miniaturize and simplify them. In U.S. Pat. No. 5,120,953 M. R. Harris teaches how to construct a confocal microscope with a single-mode transmissive optical fiber. The end-face of the fiber core provides the aperture from which the laser light issues and into which the light reflected from the object is focused or imaged. Such arrangement relaxes the geometrical constraints of the traditional confocal microscope since the end of the transmissive fiber can be placed at an arbitrary location thus defining the point source location and the image point location.
More extensive teaching on the use of transmissive fibers for carrying the light to a remotely placed photodetector and simultaneously delivering the source light is presented by S. Kimura, et al. in "Confocal Scanning Optical Microscope Using Single-Mode Fiber for Signal Detection", Applied Optics 30 (16), pp. 2143 and Juskaitis, et al. "Imaging in Reciprocal Fibre-Optic Based Confocal Scanning Microscopes", Optics Communications, Vol. 92, (4, 5, 6) pp. 315. Further details and advantages relating to confocal microscopes using various types of transmissive optical fibers are presented in U.S. Pat. No. 5,161,053 to T. P. Dabbs and U.S. Pat. No. 5,557,452 to D. W. Harris. The latter describes the advantage of using a transmissive optical fiber which is scanned in the x-y plane by use of a piezoelectric bimorph cell, which then produces a scanning image of the fiber end over the object without the use of scanning mirrors.
Auxiliary optical elements can be added to a fiber-optic based confocal microscope to make it polarization sensitive so as to perform polarization microscopy. Specifically, a transmissive birefringent optical fiber is used to transmit both, the polarized source light and the polarized return signal. The corresponding teaching is supplied by L. Giniunas, et al. in "Scanning Fiber-Optic Polarization Microscope", Optics Communications, Vol. 100, pp. 31 and P. M. F. Neilson, et al. in "Polarization-Sensitive Scanned Fiber Confocal Microscope", Optical Engineering, 35 (11), pp. 3084.
A miniature confocal microscope head for use in optical disk data storage applications is described in U.S. Pat. No. 4,626,679 issued to T. Kuwayama et al. This head uses a single-mode transmissive fiber in a reciprocal imaging arrangement. In one embodiment, a polarization maintaining optical fiber is used to transmit a linearly polarized beam from the light source to the optical head. The linearly polarized light emerging from the output facet of the fiber then passes through a quarter-wave plate located in the optical head and becomes circularly polarized. After reflection from the surface, the light passes back through the same quarter-wave plate a second time in the opposite direction as it returns to the output facet of the fiber. The returning light is linearly polarized with its plane of polarization orthogonal to the original polarization of the emerging light. Both orthogonal polarization modes are supported by the fiber without mutual interference. A polarizing beam splitter at the other end of the fiber separates the returning light from the injected light and directs the returning light to an optical-to-electrical transducer.
In another embodiment utilizing transmissive optical fibers, a miniature optical head suitable for reading a magneto-optic memory includes a polarizing beam splitter in the optical head to allow one to detect the rotation of the linearly polarized light after it is reflected back with a slight magneto-optically Kerr-induced polarization rotation. The two orthogonal components of polarized light are split off by means of the polarizing beam splitter in the head. The return light is delivered to the detection unit using two additional "return" fibers for subsequent differential detection, as is well-known in the art.
An additional refinement to a confocal system using a transmissive optical fiber is discussed by Juskaitis, et al. in "Fibre-Optic Based Confocal Microscopy Using Laser Detection", Optics Communications, Vol. 99, pp. 105. In this arrangement the light exiting through one end of the lasing cavity is coupled into a single-mode transmissive fiber for reciprocal imaging. The light returned from the object through the fiber is coupled back into the lasing cavity and alters the light intensity as well as the modes supported by the cavity. These changes are detected by a detector placed on the other end of the lasing cavity. Alternatively, when a semiconductor laser is used then the changes can be detected through the diode junction voltage. In that case, the source and the detector are integrated into one device. A discussion of this alternative can be found in R. Juskaitis, et al. "Semiconductor Laser Confocal Microscopy", Applied Optics, 33 (4), pp. 578 (1994).
An additional improvement to this confocal system, also realized by R. Juskaitis, et al. and described in "Spatial Filter by Laser Detection in Confocal Microscopy", Optics Letters, 18 (4), pp. 1135, consists of using the laser's mode confinement properties. For example, a He-Ne laser or a laser diode can be used for reciprocal imaging thus eliminating the need for a fiber optic delivery system altogether. Of course, these non-fiber based systems lack the flexibility inherent in fiber-based systems.
Several other solutions using a laser as an integrated light source and detector are also known. For example, Webb, et al. present a system with a confocally self-detecting micro laser array in U.S. Pat. No. 5,563,710. This system incorporates beam splitters and a photodetector in the optical head for detection of the laser feedback signal.
In a non-confocal arrangement, a different method of laser self-detection can be used. For example, a near-field scanning optical system which employs laser self-coupling to achieve reflective feedback detection is described in U.S. Pat. No. 4,860,276 to H. Ukita, et al. In this design, the optical data storage head uses a semiconductor laser which does not have an output cavity mirror and which must be closely coupled to the recording surface which acts as the missing laser cavity mirror. This system attains a high signal-to-noise ratio in near-field image detection, as long as the distance between the laser and the surface does not vary more than a small fraction of the laser wavelength during a scan.
Another near-field optical microscope using self-coupling is presented by Betzig, et al. in U.S. Pat. No. 5,389,779. In contrast to transmissive fibers this microscope uses an emissive fiber or a fiber laser. The output end of the fiber laser has to be non-reflective and must be situated a distance d less than one wavelength (d&lt;.lambda.) away from the surface which is being imaged to ensure proper coupling between the laser and the surface. In addition, the cross-section of the output end has to taper down to the same dimension d. These Near-Field Scanning Optical Microscopes do not have the optical sectioning capabilities offered by confocal microscopes, where the image point may be on or within the object, as needed for imaging three-dimensional objects and surfaces. Also, systems of this type require a means for keeping the end of the fiber or laser a fixed distance within one wavelength from the surface, making fast x-y scanning impractical or impossible. In any case, the data rate is limited by such a device because only low power levels can be out-coupled from the tapered tip.
Confocal systems using transmissive fibers exploit the flexibility and miniaturizability afforded by the fibers to adapt the system to various applications. Generally speaking, however, these systems still suffer from many limitations.
First, the small core diameters of the single-mode fibers used in those systems are inefficient at in-coupling of the probe beam from a compact high power (.gtoreq.100 mW) source. For example, it is difficult to efficiently couple the beam from a high power multi-mode edge-emitting semiconductor laser into the single mode transmissive fiber. The optical power levels which can be in-coupled into a single mode fiber are low because the beams from high power semiconductor lasers typically have poor spatial distribution and relatively high M.sup.2 values. Second, the power instability of the source, e.g., mode-hopping from feedback or longitudinal mode hopping induced by temperature changes, etc., is a major factor affecting the signal-to-noise ratio (SNR). Third, the wavelength instability, also due to temperature fluctuations, causes chromatic imaging errors. Fourth, the passive nature of the transmissive fiber does not enable implementation of highly sensitive heterodyne detection methods known in the art for detecting Doppler-shifted signals for high SNR signal detection. In addition, these systems do not accommodate integration of the optical elements (for polarization, isolation, beam in-coupling, beam out-coupling, detection, etc.) of a fiber-based microscope which can provide simple and economical construction. Also, the prior art systems do not provide a source that can be conveniently adjusted to operate at multiple discrete wavelengths over the visible spectrum to take advantage of chromatic focusing using diffractive type focusing elements.
Finally, most of the prior art systems require the costly fabrication of multiple optical elements and experience optical losses because the signal beam and the return beam have to pass through several in-coupling and out-coupling surfaces or elements. Integration of the entire system including the light source would therefore be highly desirable.